Unsaturated polyester resins are useful for making molding compositions which consist of an unsaturated polyester resin, an ethylenically unsaturated copolymerizable monomer, inert mineral fillers, fibrous reinforcing fillers and a catalyst which initiates the cross-linking reaction between the copolymerizable monomer and the unsaturated polyester resin at a chosen molding temperature. They are commonly used in many physical forms, including bulk molding compounds and sheet molding compounds. The use of unsaturated polyester resins in the molding of glass fiber reinforced or other fiber reinforced products enjoys broad application in the manufacture of automotive, industrial and home products.
Unfortunately, the unsaturated polyester resins are inherently brittle and have low resistance to impact. Attempts have been made to improve the brittleness thereof, but with limited success. It has been the normal industry practice to incorporate a flexibilizer in the resin composition, however, this is disadvantageous in that it results in reduction in the mechanical properties thereof which renders it unacceptable in many applications. More recently, inflexibility and low resistance to impact have been improved by blending reactive liquid polymer tougheners, such as vinyl terminated liquid polymers or hydroxyl terminated liquid polymers, with the unsaturated polyester resin before curing. Utility of such toughening agents is limited, however, by their incompatibility with the polyester resins. This is especially true of non-liquid high molecular weight elastomers. Hydroxyl terminated polyepichlorohydrin liquid polymers appear to have better compatibility with the unsaturated polyester resins. Due to the incompatibility between the toughening agents and the unsaturated polyester resins, it has been necessary to blend the two components shortly before the compounding step in the preparation of molding compositions based on polyester resin as the binder. If a blend of a toughening agent and unsaturated polyester resin were allowed to stand for a period on the order of a few days, the components would separate and the blend would have to be reconstituted again.
It has been discovered that compatibility of the toughening agents and the unsaturated polyester resins can be improved by co-reacting reactive liquid polymer tougheners with the reactants to produce unsaturated polyester resin in order to introduce the reactive liquid polymers into the backbone of the unsaturated polyester resins. In fact, it has been discovered that incorporation of the reactive liquid polymers into the backbone of the unsaturated polyester resins improves toughness properties without adversely affecting any physical or mechanical properties of the unsaturated polyester resins compared to systems where the two components are blended.